The potential energy of a triatomic molecule is approximated as follows:

U(x₁₂ , x₁₃ , θ) = 0.5 K (x₁₂ - x0₁₂)² + 0.5 K (x₁₃ - x₀₁₃)² + 0.5 K_θ (θ - θ₀)²

This energy surface ensures that equilibrium geometry of the triatomic molecule is the configuration with least potential energy. The equilibrium geometry is obtained when θ = θ₀, x₁₂ = x₀₁₂ and x₁₃ = x₀₁₃.

The internuclear forces can be determined by calculating the negative gradient of U(x₁₂ , x₁₃ , θ) with respect to the atomic coordinates of the atoms that are not fixed and their positions are controlled using the optical tweezer.